Recently, there has been proposed a vehicle's stabilizer system including a left and a right stabilizer bar whose rolling-restraining effect is changeable. For example, International Patent Application Publication WO99/67100 or its corresponding Japanese Patent Application Publication 2002-518245 discloses a stabilizer system including a pair of stabilizer bars and an actuator that rotates the two stabilizer bars relative to each other so as to change a rolling-restraining effect thereof to restrain the rolling of vehicle's body.
In the above-indicated stabilizer system, an electric current supplied to the actuator is controlled so that the two stabilizer bars may exhibit an appropriate rolling-restraining effect. Otherwise, an appropriate rolling-restraining effect may be exhibited by controlling an amount of relative rotation of the two stabilizer bars from a reference relative-rotation position thereof, to a target relative-rotation amount. In the latter case, however, the relative-rotation amount of the two stabilizer bars from the reference relative-rotation position may not be appropriately controlled to the target relative-rotation amount because of various reasons. For example, even if a relative-rotation amount of the two stabilizer bars when the power of the vehicle is turned off may be stored in a memory, the stored relative-rotation amount may differ from a relative-rotation amount of the two stabilizer bars when the power of the vehicle is turned on again. In addition, for example, because an air pressure in one of a left and a right wheel of the vehicle is too low, a posture of the vehicle when it runs straight may be tilted in a rolling direction. In those cases, an appropriate relative-rotation amount of the two stabilizer bars may not be obtained and accordingly an appropriate rolling-restraining effect may not be exhibited.
The above-indicated problems are just examples of the problems that do not allow the conventional stabilizer systems to exhibit an appropriate rolling-restraining effect. That is, the conventional stabilizer systems should be improved with respect to various points, e.g., the stability of posture of the vehicle with respect to the rolling direction.